1. Field of the Invention
The present invention relates to the real-time recording and playback of information in the form of minute, optically-readable deformations which are spaced along spiral or concentric tracks on a disc-shaped storage medium and to an improved disc construction which facilitates such recording and playback.
2. Description of the Prior Art
Optical discs of the kind described above offer the advantage of increased information capacity over previously used information-storage media such as magnetic tape and microfilm. In general, the optical disc approach involves forming micron-sized information bits (e.g., light-modulating discontinuities) along recorded tracks on a surface of the disc. One common way to form such discontinuities in real-time (i.e., so they are readable without an intermediate processing procedure) is to scan the disc with a focused laser beam which is turned on and off according to an encoded signal that contains the information to be recorded. The laser thus forms tracks of discrete deformations in the disc surface. On playback, the tracks are illuminated by a tightly focused reading light beam and variations in the light from deformed and non-deformed track portions are sensed by a photodetector to reproduce the encoded signal.
Although the disc configuration described above is currently the most popular format for such optical storage media, at this stage it should be noted that there are useful formats other than the disc. For convenience, this discussion will refer to the storage media as optical discs, with the understanding that there are other formats having equivalent utility in accordance with the present invention.
To maximize information capacity of optical discs, it is desirable to form the information tracks as closely together as possible without creating a likelihood for interference between adjacent tracks. Interference between tracks can be thought of as a deformation from one track extending sufficiently toward an adjacent track to be detected during the read-out scanning of the adjacent track. Because of this problem track spacing has been limited as to closeness by the minimum size deformation that can be recorded and by the accuracy with which such deformations can be placed as tracks on the recording medium.
Even though high numerical aperture lenses are used to focus the recording energy accurately onto the minute locus of the intended deformation, fluctuations will occur in the focused energy density. For example, recording laser noise, focus servo errors or variations in recording media transport rate can cause such fluctuations. These fluctuations can lead to undesired variations in cross-track width of a deformation. The recording material's sensitivity to the recording energy can also vary and cause undesired cross-track width variations.
Even more troublesome is the difficulty in effecting the recording spot scan precisely along a predetermined path, for example, concentric circles or convoluting spiral tracks. That is, the degree of sophistication and cost of playback tracking is severely aggrevated if the recorded tracks are not highly precise in configuration. Complex and expensive control systems are necessary in order to record tracks precisely in their intended positions.
It will be appreciated that for reasons similar to those described above, the minimum track spacing which can be utilized also depends on the size of the reading scan spot, its size variation and the degree of accuracy with which it can be maintained centered on the given track during read-out.
Thus, to obtain close track spacing and the resultant high data storage density, it is necessary to control many parameters precisely. The cost for such control equipment is high; therefore, techniques for relaxing the parameter tolerances and simplifying such control are continuing development objectives.
British Pat. Nos. 1,465,742 and 2,016,747 disclosed one technique for simplifying recording control. In general these patents suggest the provision of some pre-formed disc track which can be followed during recording. These discs are intended as masters for forming replicates. In discs intended for real-time recording and playback, the pre-formed guide track disclosed in those patents would cause noise during playback of the recorded information signal.
In my U.S. Pat. No. 4,176,377, there is disclosed a technique which uses a disc that is sensitive to radiation only along pre-formed recording tracks, and intervening non-sensitive guard bands are located between the recording tracks. Such disc surfaces are pre-formed, e.g., by thermally scribing away the intertrack zones of a recording layer or by photolithographic formation of the recording layer so as to exist only on the predetermined track paths.